Have you ever realised that, all your life,
you've missed something that's kind of obvious?
It doesn't have to be something important. It might just be really minor,
but maybe someone says a word
and your brain just kinda shifts a little
as you realise that: you've never thought of that,
and it sort of feels like you should have.
Last time I was making a video here at this explosives test facility.
I just casually used the phrase "high explosives"
because I think I assumed that "high explosive"
just meant really, really explosive.
So I was surprised when Steve, the explosives expert, pointed out
that it is a technical term and there is also "low explosive".
So the obvious thing to do was to demonstrate both.
- Explosives are a mixture of a fuel and an oxidizer.
With low explosives, these are roughly mixed together.
So they're particles of the two, in close proximity.
When a low explosive reacts, it's a thermal reaction.
So it's heat travelling through the low explosive
causing the chemical reaction to occur.
As it burns the fuel and the oxidizer react together, producing lots of gas
that will just bleed off into the atmosphere
as a big puff of smoke.
In order to make a low explosive have a more violent reaction,
you need to contain it.
So you need to hold those gases in and allow the gas pressure to build up
until it bursts through the container that you've put it in.
So you've got deflagration,
which is when you hold all the material in, and the gas pressure,
so the pressure builds up, which creates more heat
which makes the low explosive react even faster.
It's a runaway reaction, and eventually
the amount of gas produced overcomes the container and bursts it open
making a bang.
- So a low explosive, just like any other fuel,
is a Thing that Burns. It just burns quickly.
The gas that it releases expands and builds up pressure and then...
- A high explosive is also a mixture of a fuel and oxidizer
but it's not actually a mixture.
It's down on the molecular level.
So you've actually got the atoms of material
in such close proximity that they're linked together within the molecule.
And because they're so close together they can react a whole lot faster.
So rather than it being a thermal reaction that propagates through,
it's actually a shockwave reaction.
It's so fast that the shockwave can cause the chemical reaction to occur.
The chemical reaction produces energy which then sustains the shockwave
driving it on even faster
and that's why high explosives can actually go
off at between 6,000-9,000 metres per second
which is pretty damn fast.
Because high explosives require a shockwave to detonate properly,
they can be exposed to heat and flame and will normally just burn away.
In fact, you're actually allowed to have 50 kilos of high explosive in your car
without having to put a hazard symbol up on your vehicle at all
because it's considered not to be that big of a problem.
Obviously when the fire brigade see you pinned
behind the steering wheel on your car at the side of the road, and it's on fire,
they're going to stand back and sell tickets
rather than actually fighting the fire.
So what you use is a detonator to give it a shockwave, to get it going.
A detonator itself is a little metal tube.
Inside there, there's a small amount of secondary high explosive,
a tiny amount of primary explosive which is set off very easily,
and then there's some sort of initiation system
that will then set off that primary explosive.
With electrical detonators, that's an electrical fuse head
and you pass a current through the fuse head
that produces a flash as it burns out.
That sets off your primary explosive
which applies a shockwave into your small amount
of secondary explosive, which amplifies the effect.
That then bigger shockwave then transfers into your main charge,
which is then sufficient to set off the main block of explosive.
- Which means, weirdly, that high explosives can be safer than low explosives.
Despite the name, if there's an accident around a load of low explosives
they're all going to catch fire and go up.
But around high explosives, as long as you keep the primaries separate,
like you're supposed to,
it takes a lot to make a high explosive detonate
or sometimes even to burn.
That is until you want them to.
- Stand by for firing.
Firing now.
- One dead GoPro. - Yep. Yep.